1. Field of the Invention
The present invention relates to a method for quantitative analysis of a metal element such as lead contained in a resin material.
2. Description of the Related Art
In recent years, legal regulations have been enforced over the world, with attention paid to the harmful properties of chemical substances. In the European Community, a legal regulation of Restriction of Hazardous Substances (RoHS) for restricting the use of specified substances was enforced in 2006 so as to regulate clearly that six kinds of harmful substances including lead (Pb) and cadmium (Cd) must not be contained in any electrical or electronic appliances. These legal obligations must be obeyed by not only manufacturers but also enterprises exporting electrical and electronic appliances to Europe. Under the circumstances, it is a serious responsibility that must be assumed by enterprises to grasp accurately the amount of the harmful substance contained in the electrical and electronic appliances and to certify that substances harmful to the environment are not contained in the electrical and electronic appliances.
Known methods of measuring Pb and Cd, which are harmful to the environment, include, for example, BSEN-1122 (2001) (Determination of Cadmium wet decomposition method), which is employed in Europe, and EPA method B (Acid digestion of sediments, sludge, and soils), which is an official regulation by the U.S. Environmental Protection Agency. In exporting electrical and electronic appliances to Europe and the U.S.A., it is necessary to measure individually the element specified in these official regulations.
The known methods for measuring the specified element includes, for example, a wet chemical analytical method in which an aqueous solution of an oxidizing acid such as nitric acid, hydrochloric acid, sulfuric acid, or hydrogen peroxide is added to a sample so as to decompose the inorganic material or the organic material forming the matrix of the sample. Also, the sample in the form of a solution is measured in general by inductively coupled plasma emission spectrometry or an atomic absorption method.
Also, an ashing method using a platinum crucible is proposed as a method for the quantitative analysis of lead in ISO6101-2 Rubber “Determination of metal content by atomic absorption spectrometry”.
However, various problems arise in the conventional official regulations referred to above, as pointed out in the following:
(1) The methods specified in the official regulations necessitate troublesome analytical operations, and a long time is required for the decomposition so as to make the methods inefficient.
(2) Since sulfuric acid or hydrochloric acid is used in the wet acid decomposition method, lead is partly converted into lead sulfate or lead chloride. In other words, loss of lead is generated, with the result that the measured value is not reliable accurate.
(3) In the ashing method using a platinum crucible, Pb or Cd tends to be lost by the rapid heating so as to lower the accuracy of the analysis.
To be more specific, where the sample is formed of a resin material, particularly, polyvinyl chloride resin containing a large amount of chlorine (Cl), the chlorine contained in the resin is combined with the metal element that is to be measured so as to form a chloride, which tends to be evaporated easily.
Also, in the case of using a platinum crucible, a serious problem arises because platinum forms a solid solution with lead (Pb) that is to be measured. Specifically, Pb is adsorbed on the wall of the platinum crucible. Also, Pb is eluted as an impurity from the platinum crucible, or platinum itself is eluted so as to form a solid solution with Pb to be measured. As a result, an isotope interference (influences of platinum oxides such as 206PtO and 208PtO given to lead isotopes of 206Pb, 208Pb) is brought about in the measuring stage by the inductively coupled plasma mass spectrometry (ICP-MS), with the result that the accuracy of the analysis is lowered.